1. Field of the Invention
The present invention relates to an optical pickup device that forms an optical spot on an information recording surface of optical recording media having different specifications, such as disk thickness and use wavelength, and more particularly, to an optical pickup device that includes an objective lens compatible with a plurality of optical disks having different specifications.
2. Description of the Related Art
High density recording of information, such as video, audio, and data information, is generally accomplished using various recording mediums, including optical disks, cards and tapes. The optical disk, which is the most commonly used recording medium, is comprised of a plastic or glass medium that has a uniform thickness in a direction axial with an incident reading or writing light, and an information recording surface on which information is recorded. Various types of optical disks have been recently developed and include a laser disk (LD), a compact disk (CD), and a digital versatile disk (DVD). Current commercialized high-density optical disk systems heighten recording density by enlarging a numerical aperture of an objective lens using a short-wavelength light source of 635 nm or 650 nm, for example. As a result, the optical disk system is able to both record and reproduce information from a DVD and reproduce information recorded on a CD, despite the fact that a CD generally does not have the same thickness as a DVD. However, due to recording characteristics of a CD-recordable (CD-R) recording medium, such a high-density digital optical disk system has to use light that has a wavelength of 780 nm in order to use current CD-R mediums having CD patterns. In order to provide compatibility between a DVD and a CD-R medium, it is very important to enable a single optical pickup to use light that has a wavelength of 780 nm and light that has a wavelength of 650 nm.
As a result of changes in the thickness of the optical disk and variations in the wavelength and numerical aperture, optical aberration occurs when an optical disk that does not have the 780 nm and 650 nm specifications is loaded in current high-density optical disk systems. For this reason, intense research is currently taking place to develop an optical pickup that both alleviates optical aberration and is compatible with optical disks that have different respective specifications. As a result, optical pickup devices that are compatible with optical disks that have different respective specifications are being manufactured.
A conventional optical pickup which is compatible with a DVD and a CD-R medium will be described below with reference to FIG. 1.
FIG. 1 illustrates an existing optical pickup that uses a conventional objective lens. The optical pickup illustrated in FIG. 1 uses a laser light having a wavelength of 635 nm during reproduction of a DVD 8, and uses a laser light having a wavelength of 780 nm during recording and reproduction of a CD-R medium 9. The laser light that has a wavelength of 635 nm is emitted from a light source 1, such as a laser diode, incident to a collimating lens 2. The laser light beams from the light source 1 are illustrated by solid lines in FIG. 1. The collimating lens 2 makes the incident light parallel light. The parallel light that passes through the collimating lens 2 is reflected by a polarization beam splitter 3, and then proceeds to an interference filter type prism 4.
At the same time, light having a wavelength of 780 nm is emitted from a light source 11, such as a laser diode, and passes in turn through a collimating lens 12, a beam splitter 13 and a converging lens 14. The light then proceeds to the prism 4. The laser light beams from the light source 11 are illustrated by dotted lines. The prism 4 totally transmits the light that has a wavelength of 635 nm which is incident after being reflected by the polarization beam splitter 3, and totally reflects the light that has a wavelength of 780 nm which is converged by the converging lens 14. As a result, the light emitted from the light source 1 is incident to a wave plate 5 in the form of parallel light formed by the collimating lens 2, and the light emitted from the light source 11 is incident to the wave plate 5 in the form of diverging light formed by the converging lens 14 and the prism 4. The light transmitting the wave plate 5 is incident to an objective lens 7 after passing through a thin-film type variable iris 6.
During recording and reproduction of information on the DVD 8, which typically has a thickness of 0.6 mm, light having a wavelength of 635 nm passes through the variable iris 6 and is focused by the objective lens 7 on an information recording surface of the DVD 8. As a result, the light reflected from the information recording surface of the DVD 8 contains the information recorded on the information recording surface. The reflected light transmits through the polarization beam splitter 3 and is detected by an optical detector 10.
The objective lens 7 also focuses the light having a wavelength of 780 nm that has passed through the variable iris 6 onto the information recording surface of the CD-R medium 9, which typically has a thickness of 1.2 mm. However, spherical aberration occurs due to the different thicknesses of the DVD 8 and the CD-R medium 9, which causes the information recording surface of the CD-R medium 9 to be located further from the objective lens 7 along its optical axis than the information recording surface of the DVD 8. When the variable iris 6 is used, which will be described later in reference to FIG. 2, the light that has a wavelength of 780 nm forms an optimal-sized optical spot on the information recording surface of the CD-R medium 9. The light that has a wavelength of 780 nm reflected from the CD-R medium 9 is reflected by the beam splitter 13 and is then detected by an optical detector 15.
The variable iris 6 of FIG. 1 has a thin-film type structure which can selectively transmit the light incident to an area identical to a diameter of the objective lens 7. For example, as illustrated in FIG. 2, the area has a numerical aperture (NA) not more than 0.6. The variable iris 6 is segmented into a first area 1 which transmits both the light having a wavelength of 635 nm and the light having a wavelength of 780 nm, and a second area 2 which totally transmits the light having a wavelength of 635 nm and totally reflects the light having a wavelength of 780 nm. The first area 1 has a numerical aperture not more than 0.45, and the second area 2 is an outer area of the first area 1. The first area 1 is also formed of a quartz (SiO.sub.2) thin film to remove optical aberration occurring in the second area 2, which is formed of a dielectric thin film. Using the variable iris 6, the light having a wavelength of 780 nm that is transmitted through the first area 1 that has a numerical aperture not more than 0.45 forms an optical spot that is appropriate for the information recording surface of the CD-R medium 9. As a result, when a loaded optical disk is changed from a DVD 8 to a CD-R medium 9, the optical pickup of FIG. 1 can be compatibly used with an optimized optical spot.
However, it is necessary for the above-described optical pickup of FIG. 1 to achieve a finite optical system with respect to the light having a wavelength of 780 nm, in order to remove spherical aberration that occurs due to compatibility between the DVD and the CD-R medium. Accordingly, the optical system becomes complicated, making it difficult to assemble optical components. In addition, an optical path difference occurs between the light passing through the first area 1 with a numerical aperture not more than 0.45, and the light passing through the second area 2 with a numerical aperture not less than 0.45, due to a dielectric thin film which is formed in the second area 2 in the variable iris 6. To remove this optical path difference, it is necessary to form a particular optical thin film, such as a quartz thin film, on the first area 1. For this reason, a quartz thin film is formed on the first area 1 and a multi-layered thin film is formed on the second area 2, which causes the manufacturing process to become complicated. In addition, the thickness of the thin film has to be adjusted on a micrometer (.mu.m) scale, which does not fit mass-production. Even though a direct overwrite technique for performing a reproduction operation at the same time as when performing a recording operation is known which involves a technique, using light beams of different respective wavelengths and a single lens, the known direct overwrite technique cannot be applied to disks of different respective specifications in order to perform recording and reproduction operations.